Isabel is a PhD student at the Terahertz and Millimetre Wave Laboratory
Opponent will be: Prof. Peter Bøggild, Department of Physics, Technical University of Denmark
Examiner: Prof. Niklas Rorsman
Main supervisor: Senior Researcher Andrei Vorobiev
As you enter the meeting, please make sure that your username reflects your actual full name for easy recognition.
During the last decades, significant efforts have been made to exploit the excellent and
promising electronic properties exhibited by field-effect transistors (FETs) with two-dimensional electron gas (2DEG) channels. The most prominent representatives of this
class of devices are high-electron-mobility transistors (HEMTs) and graphene field-effect
transistors (GFETs). Despite the relative maturity of the HEMTs and considerable efforts
recently applied to develop the GFETs, a better understanding of the charge carrier
transport mechanisms is required for their further development. This thesis work is
focused on studying the charge carrier transport in the InGaAs/InP HEMTs and GFETs
using the geometrical magnetoresistance (gMR) effect.
The angular dependencies of output characteristics of the InGaAs/InP HEMTs oriented in a magnetic field (B) up to 14 T at 2 K were investigated. A strong angular
dependence as a function of the B-field was identified. It was shown that the gMR effect
governs the observed performance of the HEMTs, and the measured dependencies were
accurately described by gMR theory. Additionally, the carrier velocity in InGaAs/InP
HEMTs was studied using the gMR effect in the wide range of the drain fields at a
cryogenic temperature of 2 K. The velocity peak was observed experimentally for the
first time, and it was found that the peak velocity and corresponding field decreased sig-nificantly with the transverse field. The relevant scattering mechanisms were analyzed,
and it was further demonstrated, that the low-field mobility and peak velocity reveal
opposite dependencies on the transverse electric field, indicating the difference in carrier
transport mechanisms dominating at low and high electric fields.
It was demonstrated, that the mobility in the GFETs can be directly characterized
and analyzed using the gMR and that the method is free from the limitations of other
commonly used approaches requiring an assumption of constant mobility and knowledge
of the gate capacitance. This allowed for interpretation of the measured dependencies of
mobility on the gate voltage, i.e., carrier concentration, and identifying the corresponding
scattering mechanisms. The charge carrier transport in the GFETs, characterized using
the gMR method in combination with the drain-source resistance model, was also studied
by applying a model of the quasi-ballistic charge carrier transport and transfer formalism.
The charge carrier mean free path was found to be in the range of 374-390 nm. GFETs
with a gate length of 2 µm were shown to have ≈ 20 % of the charge carriers moving
ballistically, while at the gate length of 0.2 µm this number increases to above 60%.
Keywords: high-electron-mobility transistor, graphene field-effect transistor, low noise
and high frequency applications, geometrical magnetoresistance, charge carrier transport, charge carrier scattering mechanisms, velocity saturation, velocity peak, low-field
mobility, two-dimensional electron gas, quasi-ballistic charge carrier transport
Kollektorn, lecture room, Kemivägen 9, MC2-huset